1. Field of the Invention
This invention relates to an automatic equalizer and, more particularly, to an automatic equalizer for equalizing, by a predetermined equalizing characteristic, received data having a data transmission interval T.
2. Description of the Related Art
A modem of a facsimile apparatus or various types of data terminals capable of performing high-speed (4800 bps) data communication using an analog line such as a telephone line usually is provided with an equalizer for eliminating signal distortion caused by the transmission line and performing equalizing by following up a change in such distortion with the passage of time.
FIG. 8 illustrates the construction of the equalizer section of a conventional modem.
In FIG. 8, the portion enclosed by the dashed line represents a transversal filter of an automatic equalizer. The transversal filter comprises a plurality of cascade-connected data delay elements 300 (each having a unit time delay of T seconds), multipliers 301 for multiplying items of delay data (R.sub.0 .about.R.sub.6) by respective tap coefficients 302 (C.sub.0 .about.C.sub.6), and an adder 303 for calculating the sum total of the products provided by the aforementioned multipliers. The adder 303 produces an output Z.sub.k, which is indicative of the sum total calculated. This signal becomes the output of the transversal filter.
In a modem having such a transversal filter, training data, which is predetermined between the transceiving apparatus, is received to set the tap coefficients before a data transmission is made. This is followed by making the transition to data transmission.
During the transmission of the training data and the transmission of ordinary data, the automatic 5 equalizer updates the tap coefficients 302 (C.sub.m : C.sub.0 .about.C.sub.6) and generates equalizer characteristics for the purpose of eliminating distortion in accordance with the following equation: EQU C.sub.m =C.sub.m -.alpha.R.sub.m *.multidot.E.sub.k ( 1)
where
.alpha.: corrective coefficient PA1 R.sub.m *: complex conjugate of input signal R PA1 E.sub.k : error signal (E.sub.k =Z.sub.k -A.sub.k)
Here the error signal E.sub.k is the difference between the adder output Z.sub.k and the result A.sub.k of discrimination performed by a decision unit 310. The value of A.sub.k is known during the aforementioned training. Reference numeral 305 denotes a sampling switch for sampling an input signal, which comprises an interval of less than T seconds, upon subdividing the signal at the time delay of T seconds of the equalizing delay elements.
An important factor which decides the characteristics of the automatic equalizer is timing phase error. Timing phase error will be described in simple terms with reference to FIG. 9.
FIG. 9 illustrates the transmission waveform of a baseband. The timing of the vertical lines represented by the solid lines indicates data points actually transmitted. The spacing between neighboring solid vertical lines corresponds to the data transmission interval (the so-called baud rate) T.
On the receiving side, this waveform is received and equalized. In actuality, however, data is not received precisely at the normal T interval as indicated by the solid vertical lines but often is received at a timing offset from the normal timing, as indicated by the dashed vertical lines. This offset .DELTA.t from the normal timing is the aforementioned timing phase error.
Since the equalizer (whose unit time delay is T seconds) shown in FIG. 8 equalizes this timing phase error as well, it goes without saying that equalization precision is greatly affected by the amount of phase error.
In general, equalization precision declines as the timing phase error .DELTA.t increases. It is known that reliable equalization can no longer be performed when the timing phase error attains a value of T/2.
In order to eliminate the drawback in the prior art, timing phase control means is provided for extracting a timing signal from the received signal and synchronizing equalization processing to the transmission timing. However, in order to extract a timing signal from transmitted data and control the phase of reception timing reliably, the control means required is of considerable complexity.